Recent anti-cancer strategies are based on the stimulation of anti-tumor immune reaction, exploiting distinct lymphocyte subsets. Among them, γδ T cells represent optimal anti-cancer candidates, especially in those tissues where they are highly localized, such as the respiratory or gastrointestinal tract. One important challenge has been the identification of stimulating drugs able to induce and maintain γδ T cell-mediated anti-cancer immune response. Amino-bisphosphonates (N-BPs) have been largely employed in anti-cancer clinical trials due to their ability to upregulate the accumulation of pyrophosphates that promote the activation of Vγ9Vδ2 T cells. This activation depends on the butyrophilin A family, which is crucial in contributing to Vγ9Vδ2 T cells stimulation but is not equally expressed in all cancer tissues. Thus, the clinical outcome of such treatments is still a challenge. In this viewpoint, a critical picture of γδ T cells as effective anti-cancer effectors is designed, with a specific focus on the best immune-stimulating therapeutic schemes involving this lymphocyte subset and the tools available to measure their efficacy and presence in tumor tissues. Some pre-clinical models, useful to measure γδ T cell anti-cancer potential and their response to stimulating drugs, therapeutic monoclonal antibodies, or bispecific antibodies are described. Computerized imaging and digital pathology are also proposed as a help in the identification of co-stimulatory molecules and localization of γδ T cell effectors. Finally, two types of novel drug preparation are proposed: nanoparticles loaded with N-BPs and pro-drug formulations that enhance the effectiveness of γδ T lymphocyte stimulation.

Until now, despite 30 years of intensive work, the RV144 human immunodeficiency virus (HIV) vaccine trial initiated in 2003 remains so far the most protective vaccine prototype of all those tested (32% reduction in the infection rate three years after the vaccination) and the HIV epidemic is still spreading worldwide. In addition, antiretroviral therapy (ART) for people living with HIV is given for life as no other pharmacological intervention has allowed to maintain an undetectable viral load after ART withdrawal. Pr Andrieu and colleagues discovered tolerogenic CD8+T-cells that suppress simian immunodeficiency virus (SIV) specific activation, ensuing SIV reverse transcription suppression and viral replication-defective in Chinese macaques vaccinated by intragastric route with inactivated SIV particles + Lactobacillus rhamnosus. Moreover, in HIV-infected elite controllers with specific genetic features (HLA-1-Bw4-80i and KIR3DL1 genes), Pr Andrieu found out that similar tolerogenic CD8+T-cells suppress in the same manner HIV-specific activation, HIV reverse transcription, and HIV replication. These data justify the development of a tolerogenic vaccine composed of inactivated HIV particles + Lactobacillus rhamnosus that could be used as a preventive or therapeutic vaccine.

Although a large number of preventative human immunodeficiency virus (HIV) vaccine trials have been carried out during the last 30 years, it is remarkable that an effective HIV vaccine has not yet been developed. Research paradigms correspond to theoretical assumptions and particular strategies that scientists use when they try to solve a particular problem. Many paradigms used successfully in vaccinology were ineffective with HIV. For instance: 1) The structure-based reverse vaccinology approach failed because investigators tried to generate a vaccine starting with the antigenic structure of HIV-envelope (Env) epitopes bound to neutralizing monoclonal antibodies (mAbs) derived from HIV-infected individuals. They assumed that this antigenic structure would also possess the immunogenic capacity of inducing in vaccinees a polyclonal antibody (Ab) response with the same neutralizing capacity as the mAb. 2) The structures observed in epitope-paratope crystallographic complexes result from mutually induced fit between the two partners and do not correspond to the structures present in the free molecules before they had interacted. 3) The affinity-matured neutralizing mAbs obtained from chronically infected individuals did not recognize the germline predecessors of these Abs present in vaccinees. 4) The HIV p17 matrix protein that lines the inner surface of the viral membrane is one of the most disordered proteins identified on our planet and this prevents the induced Abs from binding to the glycosylated HIV gp120 protein. 5) Vaccinologists need to solve so-called inverse problems, for instance, guessing what are the multiple causes that produced an earlier wanted beneficial effect such as the absence of deleterious HIV infection in elite controllers. Since the immune system consists of numerous subsystems that have not yet been elucidated, it is impossible to solve the inverse problems posed by each subsystem. 6) Vaccinology is an empirical science that only sometimes succeeds because we do not understand the complex mechanisms that lead to protective immune responses.

Gamma delta lymphocytes (γδ T) sit at the interface between innate and adaptive immunity. They have the capacity to recognize cancer cells by interaction of their surface receptors with an array of cancer cell surface target antigens. Interactions include the binding of γδ T cell receptors, the ligands for which are diverse and do not involve classical major histocompatibility complex (MHC) molecules. Moreover, a variety of natural killer-like and fragment crystallizable gamma (Fcγ) receptors confer additional cancer reactivity. Given this innate capacity to recognize and kill cancer cells, there appears less rationale for redirecting specific to cancer cell surface antigens through chimeric antigen receptor (CAR) expression. Several groups have however reported research findings that expression of CARs in γδ T cells can confer additional specificity or functionality. Though limited in number, these studies collectively identify the potential of CAR-T engineering to augment and fine tune anti-cancer responses. Together with the lack of graft versus host disease induced by allogeneic γδ T cells, these insights should encourage researchers to explore additional γδ T-CAR refinements for the development of off-the-shelf anti-cancer cell therapies.

Allogeneic stem cell transplantation is currently the only curative approach for a variety of malignant and non-malignant diseases. In the early transplant era, the intent of this treatment was to apply an intensive myeloablative regimen to eliminate residual malignant cells followed by the hematopoietic rescue of the patients with donor hematopoietic stem cells. However, the focus has shifted over time and allogeneic transplantation is nowadays seen as a cellular therapy in which the donor-derived immune system mounts an anti-infectious and especially an anti-tumor effect in the posttransplant phase. In order to further augment the anti-tumor effect, various approaches have been developed, including the manipulation of the donor-derived immune system in vivo or the adoptive transfer of ex vivo-expanded donor-derived effector cells. Based on their lack of alloreactivity, γδ⁺ T cells are shifting into the spotlight of research in the context of allogeneic transplantation. Their exploitation with regard to their anti-infectious and anti-tumor properties and their in vivo and ex vivo manipulation will lead to new therapeutic approaches to improve the outcome of patients after allogeneic stem cell transplantation. In this review, the important role of γδ⁺ T cells in allogeneic matched and mismatched transplantation is summarized and an outlook is discussed on how to best make use of this unique cell population.

Avidity of immunoglobulin G (IgG) is defined as its binding strength to its target antigen. As a consequence of affinity maturation of the IgG response, avidity is maturing as well. Therefore, acute infections are characterized by low-avidity IgG, whereas past infections are usually associated with high-avidity IgG. Avidity maturation is also observed as a consequence of optimal vaccination. Avidity has been shown to play a significant role in protective humoral immunity in many microbial systems. After severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, the situation is different compared to other viral infections, as the moderate degree of avidity reached in most cases of infection is similar to that reached after only one vaccination step. In contrast, two vaccination steps lead to a much higher avidity of IgG directed towards viral spike protein S1 (S1) in the majority of vaccinated individuals. Therefore, it seems that two vaccination steps allow for a more extended affinity/avidity maturation than natural infection. The degree of avidity maturation after two vaccination steps is heterogeneous. It can be further enhanced by a third vaccination step. Complete avidity maturation seems to depend on sustained availability of antigen during the maturation process. Variants of concern seem to increase the affinity of their receptor-binding domain (RBD) to angiotensin-converting enzyme-2 (ACE2) and/or to decrease the susceptibility for neutralizing antibodies. Classical neutralization tests do not necessarily reflect the avidity of neutralizing IgG, as they operationally dissect the binding reaction between S1 and IgG from the binding of the S1 to ACE2. This approach fades out critical competition reactions between IgG and ACE for RBD of the S1. Quantitative avidity determination might be an essential tool to define individuals that only possess suboptimal protective immunity after vaccination and therefore might benefit from an additional booster immunization.